The natural lens of an eye can lose transparency and become opaque, forming what is called a cataract. The cataract can vary greatly in size and opacity. If the cataract is large enough and sufficiently opaque to affect vision significantly, it is removed surgically.
Removal can be by means of removing the lens and the capsular bag in which it rests or by removing the lens through the anterior face of the capsular bag, leaving the posterior portion of the bag intact. This latter method is called extracapsular extraction, and it is preferable in many cases because it reduces postoperative complications.
After lens extraction, light entering the eye remains unfocused, causing blurring. A corrective lens can be used to improve vision, with the normally preferred method being implantation of an intraocular lens in the posterior chamber, either between the iris and capsular bag or in the capsular bag. A lens is implanted in a position in the eye where it is centered horizontally and vertically and where it is located in an anterior/posterior location with respect to the remainder of the eye so as to provide optimal vision. Finally, it is desirable to position the lens vertically with respect to the retina, with a minimum of tilting or tipping.
Most IOLs are equipped with thin flexible haptics or support loops which project out from the lens optic for mounting purposes. These haptics are positioned within, and sometimes attached to, the capsular bag to hold the lens in the desired position within the eye. Flexible haptics are generally preferred because they adapt to changes in the shape and size of the eyeball without becoming dislodged or causing trauma to surrounding tissue.
IOLs having thin, flexible haptics are more likely to move slightly away from their desired position than are lenses having stiff haptics. This undesirable dislocation can be in the vertical or horizontal mode, called decentration or tilt, or it can be in the anterior or posterior direction.
An IOL with thin, flexible haptics is also more susceptible to tipping or tilting, which is more likely to be in the anterior direction than in the posterior direction because of the presence of the posterior face of the capsular bag, limiting movement in the posterior direction. An IOL which moves in the anterior direction can in some cases move far enough to be captured by the pupil, resulting in at least temporary loss of vision.
As discussed, there are advantages and disadvantages with both flexible haptics and stiff, flat haptics. Each one has trade-offs against their advantages. It is difficult to produce an IOL which provides adaptability to all conditions in the eye without sacrificing some stability and resistance to decentration and tilting.